Our sweat glands become more active when we are nervous or excited which leads to greater perspiration. For this reason even if a criminal is very cautious, he could leave latent prints on the scene of crime. Latent prints are the prints which are not visible to naked eyes.
The visibility of a latent fingerprint depends on-
- The substrate with which contact is made
- Temperature and climatic factors
- Amount of time passed
- Physical condition of the skin of the criminal
They are developed using wide array of physical, optical and chemical methods. The method used for development depends on-
- The type of surface on which the print is suspected. It can be porous and non porous. Porous surfaces are absorbent. Eg – paper, cardboard, wood, and other forms of cellulose. Whereas non porous surfaces do not absorb. Eg – glass, metal, plastics, lacquered or painted wood, and rubber. Fingerprints present on non porous are more susceptible to damage.
- Condition of the substrate. Eg- clean, dirty, tacky, sticky, greasy, etc.
- Subsequent forensic examination to be undertaken and the consequences of the development method used.
- Amount of time elapsed
- Environmental conditions
Powder method for fingerprint development
This is one of the oldest and simplest method which is commonly used for development of latent fingerprint. Latent prints can be visualized using different kind of powders [a powder having fine particles which will physically adhere to latent print residue (on nonporous surface) is used]. Powder used tends to have affinity for moisture and adheres to the residue deposited by friction ridges.
In earlier days, practitioners used variety of material which was easily available to them for making dusting powder, like charcoal, lead powder, cigar ashes, etc.
Powder method is typically used on non porous surfaces and not recommended for absorbent or porous surfaces.
As latent fingerprint is fragile evidence, brushes used for dusting powder should have soft bristles to prevent any damage to the print. If a brush is too stiff or used with extensive force then the latent prints having high oil content or moisture can be easily damaged. Usually these brushes are made with animal hair, fiberglass filaments, or sometimes feathers.
Brushes should be kept clean, dry and tangle free.
Application of powder to the fingerprints
- First step is “loading” the brush, which means applying the fingerprint powder on the brush by lightly dipping its tip into a sterile, wide-mouth container holding a small amount of powder.
- Excess powder is removed from the brush by shaking.
- Before applying the powder to all the areas, it is applied to a small portion or to a substrate similar in nature for testing. (This is done to determine the optimum amount of powder required for that particular substrate and to prevent background painting.)
- After testing is done, the suspected area is brushed with light and even strokes just like painting.
- Once the print starts to appear, the brush is moved in the direction of ridge flow to prevent the damage to the print.
They are based on the principle of luminescence. They are created by adding laser dye to the binders and then the mixture is allowed to evaporate and finally grounding the dried mass into the latent fingerprint powder. They are commonly used on multicolored backgrounds.
Magnetic or magna powder-
This method uses the ferromagnetic properties of iron powder mixed with pigment powders. The powder is applied with the help of a magnetized rod which has no bristles. When magnetic applicator is dipped into powder it forms a magnetic powder ball which act as a brush (much softer than conventional filament brushes and causes less damage to fragile latent prints) and serves as an effective carrier of pigment particles and it is passed back and forth over the surface for development of latent prints.
Disadvantage- They are less effective on ferromagnetic substrates like steel because of the magnetic attraction which may cause contact between the applicator and substrate damaging the latent print in process.
Preservation of the powdered print
The most commonly used technique for preservation of powdered prints is tape lifting. It is an economical and easy to work with technique. In this technique, sufficient amount of tape is pulled off from the roll. A tab is made by folding a small portion from the corner to hold the tape conveniently. Then the tape is applied on the print. Make sure no air bubble is formed when the tape is being applied. Rub over gently to ensure good adhesion to the print. Thereafter pull off the tape and transfer it to a contrasting background ( eg- black fingerprint powder placed on a white backing card).
Other adhesive lifting materials are also used like hinge lifter.
While working with any type of powder it is strongly recommended to wear a dust mask or work on a downdraft table as a precaution.
Chemical Method of Development
This method is used to develop latent prints on paper and other porous substances. Our sweat contains small but detectable amounts of amino acid and amino acids have high affinity for cellulose, so when our fingers come into contact with paper a small amount of sweat (containing amino acid) get impregnated on the paper. The amount of amino acid present in the print reduces with time (for this reason it is advisable to develop print as soon as possible).
Ninhydrin is a non specific amino acid reagent and highly suitable for development of latent prints. When ninhydrin reacts with the amino acids present in the sweat it forms ruhemann’s purple along with aldehyde derivative of the amino acid and carbon dioxide as by products (Prints are developed in purple reddish brown colour). Ruhemann’s purple degrade in the presence of light and oxygen, it is advisable to store the developed print under cool and dark conditions (developed prints should be photographed before storing for record; a green filter is used for better contrast on coloured backgrounds).
DFO is used for development of prints on porous substances, primarily cardboard and paper. DFO reacts with the amino acids present in the print and gives faint red or pink fingerprints that were intensely fluorescent at room temperature. It is more sensitive reagent than ninhydrin for development of prints. It does not develop prints on surfaces that are wetted or exposed to high humidity.
Note – If we use DFO followed by ninhydrin, it gives better results than DFO and ninhydrin alone.
This method is used for developing prints on non porous surfaces. When the surface suspected to have latent print is exposed to vapor of ethyl or methyl cyanoacrylate it produce a white deposit of cyanoacrylate polymer. The white deposit can be enhanced using fluorescent dyes like Rhodamine 6G. To get rapid results, humidity should be raised 80% and glue temperature to around 120 degree Celsius (these conditions can develop old prints in about 20 minutes).
Silver nitrate method
This method is used for developing prints on porous surfaces like documents, currency, unpainted wood,etc. Silver nitrate reacts with sodium chloride present in our sweat and forms an unstable white silver chloride. When it is exposed to sunlight, silver chloride breaks into its components (silver and chlorine) giving reddish brown prints.
Iodine fuming method
The prints which contain fats or oils are developed using iodine fuming method. When iodine crystals are heated they turn into vapours without turning into liquid phase (called sublimation) and the fumes generated reacts with the fatty acids or oils present, giving yellowish brown prints. Once we stop treating the surface with iodine fumes the developed prints starts to fade away. For this reason prints should be photographed as soon as they are developed. (Iodine fumes should not be inhaled as they are harmful)
If we spray 1% solution of starch in water on developed prints, they will turn into blue and can last upto several weeks.
There are many other chemicals that can be used to develop latent prints like osmium tetroxide, chem. Print, hydrofluoric acid, tannic acid, osmic acid, mercuric iodide, fleming’s reagent, etc.